1. Field of the Invention
The present invention relates to electrochemical electrodes and to methods for the preparation thereof. More particularly, the present invention relates to porous, conductive electrodes comprising carbon particles distributed in sol-gel-derived matrices. These sol-gel-derived silica-carbon composites can be used as inert electrodes, reference electrodes, or selective electrodes and for electrochemical catalysis and biosensing. The composite silica-carbon electrodes can be produced in the form of thin layers, monolithic rods or disks, and in the form of microelectrodes.
2. Description of the Related Art
The term "sol-gel technology" as used herein is a general name for the known process of production of silica and metal oxide ceramics by meltless processes through the polymerization of suitable monomers (such as the metal alkoxides), which produce colloidal suspension ("sol") and, upon further agglomeration, produce xerogel ("dry gel") or dry film states.
The term "sol-gel glass" as used herein relates to any ceramic or organoceramic material as depicted generally in C. J. Brinker and G. W. Scherer, Sol-Gel Science, Academic Press, San Diego, Calif., U.S.A., 1990, the teachings of which are incorporated herein by reference.
The term `sol-gel process,` as used herein, is as defined and explained in C. J. Brinker and G. W. Scherer, Sol-Gel Science, discussed above, and is defined broadly as the preparation of ceramic materials by preparation of a sol, gelation of the sol, and removal of the solvent.
Most of the sol-gel techniques use low molecular weight tetraalkoxysilane precursors (mainly, tetramethoxysilane abbreviated as TMOS, or tetraethoxysilne abbreviated as TEOS), although it is also possible to use sodium silicate precursors. The overall chemical reaction is given by Equation 1: EQU Si(OR).sub.4 +(4-x)H.sub.2 O.fwdarw.SiO.sub.x (OH).sub.4-2x +4ROH(1)
The reaction proceeds through hydrolysis (Equation 2, below), and condensation ( Equation 3, below ) steps: EQU .tbd.Si--OR+H.sub.2 O.fwdarw..tbd.Si--OH+ROH (2) EQU .tbd.Si--OR+HO--Si.tbd..fwdarw..tbd.Si--O--Si.tbd.+ROH (3)
Since alkoxysilane is not miscible in aqueous solution, methanol or another solvent (e.g., THF, alcohols) is frequently used for homogenization.
Unlike the polymerization of organic polymers that is governed by the formation of chain polymers, which branch and crosslink to form the gel, silica polymerization is believed to evolve mainly through the formation of a colloidal suspension (the sol), which gels by agglomeration. Since silica oligomers are silanol rich, the pH level strongly influences the kinetics of the agglomeration and .the final structure of the xerogel. High pH conditions produce condensed particulate sols, which eventually agglomerate to give highly porous silica gels. Low pH (2-7) polymerization gives branched polymeric sols and dense, high surface area (up to about 1000 m.sup.3 /gr) gels.
During the last stage of the gelation, water and solvent evaporate from the glass cavities, thus forming the dry gel (xerogel) state.
Often a high temperature sintering step is used to densify the porous layer and to form poreless films or monoliths. This step is omitted or used mildly in the preparation of the silica carbon composite electrodes of the present invention.
Using the sol-gel process, it is possible to produce ceramics in virtually any desired configuration, including thin films, powders, fibers and monoliths, and from various metal oxides such as titania, silica, alumina, vanadium oxide, and mixed oxides of different compositions.
By using a different type of precursors, it is possible to produce modified silica matrices with controlled surface properties. For example, a mixture of methyltrimethoxysilane (MTMOS) and tetramethoxysilane (TMOS) monomers gives: EQU (1-y)Si(OCH.sub.3).sub.4 +y(CH.sub.3)Si(OCH.sub.3).sub.3 +(4-x-y)H.sub.2 O.fwdarw.SiO.sub.x (OH).sub.4-y-2x (CH.sub.3).sub.y +(4-y)CH.sub.3 OH(4)
Replacing the methyl group in the methyltrimethoxy with octadecyl or another radical ( such as phenyl, aminoalkyl or cyanoalkyl) alters the surface properties of the material (G. Philipp and H. Schmidt, J. of Non-Crystalline Solids, Vol. 63, p. 283, 1984; H. Schmidt and H. Wolter, J. of Non-Crystalline Solids, Vol. 121, p. 428, 1990).
Recently it was found, as described in pending U.S. patent application Ser. No. 07/637,873, that it is possible to entrap inorganic, organic and biological chemicals in sol-gel ceramics by incorporating them with the sol-gel precursors, and that these reagents can interact with diffusible solute or components in an adjacent liquid or gas phase. Hereinafter, these materials are referred to as "doped sol-gel glasses."